The present invention relates to methods for determining parameters that significantly effect the design of superconducting devices. More particularly, this invention pertains to an improved method for measuring transport critical current density and flux penetration depth in bulk superconductors.
Superconductors constitute a new generation of materials that promise exciting developments in various fields including transportation (e.g. propulsion systems for levitating trains), magnetohydrodynamic power (e.g. propellerless submarines), ore separation, magnetic shielding, and medical instrumentation (e.g. magnetic resonance imaging).
So-called superconducting materials are characterized by an absence of resistance to the flow of charge that characterizes electrical current. As a result, such materials potentially provide very strong magnetic properties in relatively small packages. Since large electrical currents can pass through such materials, superconductors can become extremely strong and efficient electromagnets.
Initial development of superconducting materials was hampered in terms of economic feasibility by the requirement of extremely low operating temperatures. The initial superconducting materials, such as niobium, required a degree of cooling that mandated the use of liquid helium to achieve superconductivity. More recently, high temperature superconductors (HTS) of ceramic composition have been developed that extend this temperature range to 40 degrees Kelvin and above. An example of such an HTS material is YBa.sub.2 Cu.sub.3 O.sub.7. Such ceramic materials function effectively in the presence of a liquid nitrogen cooling bath thereby achieving economies and feasibility well beyond that of the initial generation of superconductors.
Two parameters essential to the optimal design of devices fabricated of superconducting materials are transport critical current density and flux penetration depth. The critical current density, a bulk property, measures the largest current that can pass through a superconductor without any loss of superconductivity while flux penetration depth measures the penetration of the magnetic field into the material (distance at which the field has decayed to the first critical field (HCl) ) The two parameters are closely related and depend upon magnetic field when the first critical field value is exceeded. Previously, the measurement of transport critical current density has been performed either by electrically contacting the surface of the superconductor material or "contactless" magnetization techniques.
The method of passing current through attached leads inevitably leads to loss of superconductivity through Joule heating at the resistive contacts before true critical current density has been achieved. On the other hand, the contact-less magnetization technique is subject to inaccuracies when the superconductor material sampled is distorted by localized magnetization currents.